Gasoline engines emit undesirable amounts of hydrocarbons when first started. This happens because liquid gasoline can become trapped in crevices in a cold engine and not be vaporized in time to participate in combustion and because the catalytic converter does not oxidize hydrocarbons until it reaches a "light-off" temperature. This problem is exacerbated by the need to provide a fuel rich mixture when the engine is first started. One known way to reduce hydrocarbon emissions is to electrically heat part of the catalytic converter before starting the engine. This has the disadvantages of delaying starting and requiring large amounts of electrical energy at times when the battery is least able to supply such energy. Another known way to reduce hydrocarbon emissions at startup is to pass the exhaust through an adsorbent for trapping hydrocarbons. When the catalytic converter reaches operating temperature the adsorbent is flushed and the catalytic converter oxidizes the flushed hydrocarbons.
Canisters of activated carbon have been used for many years to adsorb fuel vaporized while vehicles are not operating. The canisters are purged while the vehicle is operating thereby preparing them to adsorb fuel again. A number of types of activated carbon are in current use or available made from raw materials such as wood and coal.
Vapor in a fuel tank may contain one to three grams of fuel per liter of vapor which is displaced during refueling and may escape into the atmosphere. It is well known that filling a vehicle fuel tank at normal flow rates produces sufficient pressure in the filler tube to cause vapor to pass through a canister of activated carbon where it can be adsorbed to prevent hydrocarbons from entering the atmosphere. This does not happen in current production vehicles because a bypass is provided at the filler tube and canister size is inadequate.
It is also well known that during purging activated carbon releases the most volatile species before releasing less volatile species. Activated carbon charged with vapors from gasoline first releases predominantly butanes, then releases predominantly pentanes.
Oxygen sensors in the exhaust of gasoline engines have been standard equipment for many years. After a brief warmup period they indicate the presence of oxygen in the engine exhaust whereupon the information provided is used to optimize the delivery of fuel to the engine.
The velocity of sound in air is about 330 meters per second and the velocity of sound in butanes is about 215 meters per second. In a mixture the velocity varies between these values. In a stoichiometric mixture of air and butane the velocity of sound is about 9 meters per second slower than in pure air.
A general object of this invention is to provide a means for reducing hydrocarbon emissions and improving operation of gasoline powered engines during initial startup which also overcomes certain disadvantages of the prior art.